1. Technical Field of the Invention
This invention relates generally to the field of water flood monitoring in low saline water environment in a subterranean formation. In particular, the method of the invention relates to detecting the encroachment of low salinity water into oil formations. This invention provides the means to gain valuable information about water movement within the oil reservoir.
2. Description of the Prior Art
Knowledge of oil and water saturation behind casing is crucial to mature field management. Saturation changes within the reservoir are an indication of how the oil formation is swept by water injection. By tracking saturation changes, decisions can be made to control rates from different perforation intervals within the producing wellbore for better management of sweep and recovery.
Thermal Decay Time (TDT) and Pulse Neutron Logging (PNL) are common logging techniques used to determine oil and water saturation. When the salinity of the injected water is high, chlorine, which has a large neutron capture cross section, is abundant in the water and can be easily identified with TDT or PNL log. TDT and PNL have been proven to be reliable methods for detection of saline water movement within oil reservoirs.
The Thermal Decay Time log is a record of the rate of capture of thermal neutrons in a portion of formation after it is bombarded with a burst of 14-MeV neutrons. An electronic neutron generator in a tool produces pulses of neutrons which spread into the borehole and formation. The neutrons are quickly slowed down to thermal energies by successive collisions with atomic nuclei of elements in the surrounding media. The thermalized neutrons are gradually captured by elements within the neutron cloud, and, with each capture, gamma rays are emitted. The rate at which these neutrons are captured depends on the nuclear capture cross sections which are characteristic of the elements making up the formation and occupying its pore volume. The gamma rays of capture which are emitted are counted at one or more detectors in the sonde during different time gates following the burst, and from these counts the rate of neutron decay is automatically computed providing information regarding the formation. One of the results typically displayed is the thermal decay time, which is related to the macroscopic capture cross section of the formation, which is also displayed. Because chlorine is by far the strongest neutron absorber of the common earth elements, the response of the tool is determined primarily by the chlorine present (as sodium chloride) in the formation water. Like the resistivity log, therefore, the measured response is sensitive to the salinity and amount of formation water present in the pore volume. The response is relatively unaffected by the usual borehole and casing sizes encountered over pay zones. Consequently, when formation water salinity permits, Thermal Decay Time logging provides a means to recognize the presence of hydrocarbons in formations which have been cased, and to detect changes in water saturation during the production life of the well. When salinity is low, conventional TDT logging is not reliable. In the proper environment, the TDT log is useful for the evaluation of oil wells, for diagnosing production problems, and for monitoring reservoir performance.
In fields where relatively low salinity water is used for enhanced oil recovery, TDT and PNL data will be meaningless. Sigma readings of low saline water and oil are very close, and consequently difficult to differentiate. TDT and PNL cannot be reliably used to detect low saline water displacement of oil. There is a need for a method to provide detection of movement within oil reservoirs for low salinity water.
Carbon/Oxygen Ratio (COR) logging is a technique introduced to the petroleum industry to obtain oil and water saturation independent of water salinity. However, field application of COR has yielded mixed results.
Techniques performed by hydrocarbon producers to increase the net permeability of the reservoir are referred to as “stimulation.” Essentially, one can perform a stimulation technique by injecting chemicals into the wellbore to either react with and dissolve portions of the formation, or to create further fissures in the formation. When these methods utilize acids, they are referred to as fracture acidizing (injection of acid at rates above fracture pressure to etch the faces of the resultant fractures) and matrix acidizing (injection of acid at rates below fracture pressure to dissolve flow channels in the rock or to remove scale or damage caused by drilling). Matrix acidization, as described above, is a stimulation method that is known solely for the purpose of productivity enhancement. Acid treatments are employed in all types of oil wells and occasionally in water wells.
Acids useful in such stimulation or acid treatment processes are typically extremely active, such as hydrofluoric acid. Aqueous acid solutions, acid-like fluids or fluid of similar function are commonly used to treat oil or gas wells. These solutions are useful for matrix acidization. Selection of the appropriate acid is made based upon the individual well.
It would be advantageous to provide a means for monitoring low saline water in a subterranean formation. It would be advantageous to also provide a means for such monitoring that is effective for horizontal wells and a method that is effective for vertical wells. It would be particularly advantageous to provide a method for monitoring water sweep in salinity environments when sigma readings as taken by convention thermal decay time and pulse neutron logging methods are very similar between low saline water and oil.